The present invention relates to a biaxially oriented laminated polyester film. More specifically, it relates to a biaxially oriented laminated polyester film which is excellent in lubricity, abrasion resistance and scratch resistance and is particularly useful as a base film for a magnetic recording medium.
Having excellent properties, a biaxially oriented polyester film is used in a variety of fields such as magnetic tapes, electrical appliances, photographs, metallization and packaging. Above all, due to its high strength and a high elastic modulus, it is widely used as a base film for a magnetic recording medium, e.g., a video tape, an audio tape, a computer tape and a floppy disk.
With increasing demands for high-density recording and high quality in the above fields in recent years, a polyester film which is to constitute a base film is increasingly required to have flat surface(s).
As the flatness of a film surface increases, for example in the field of magnetic tapes, the film has an increased coefficient of friction, and the tape is liable to suffer a failure in running and to undergo scratching. Further, as the flatness of a film surface increases, the form of a film roll extraordinarily deteriorates at a step of taking up the film in the form of a roll in film production, and it is difficult to prepare a film roll in a good form. Further, for higher productivity, it is required to increase the take-up rate and the film width, while it is increasingly difficult to prepare a film roll in a good form because of increases in the take-up rate and width.
A polyester film which is to constitute the above base is therefore required to be excellent not only in surface flatness but also in lubricity for preparing a film roll in a good form.
For improving the lubricity of a film, there have been proposed a method in which inorganic fine particles of silicon oxide or calcium carbonate are added to a polyester and a method in which fine particles containing calcium, lithium or phosphor are deposited in a polymerization system when a polyester is synthesized. These two methods sought to improve the lubricity of a film by forming fine protrusions derived from fine particles on the film surface when the film is formed from a polyester.
However, in the above methods of improving the film lubricity by means of protrusions formed of fine particles, generally, the lubricity improves as the roughness of the film surface increases, whereas, for example, in the field of a magnetic recording medium, the surface of a magnetic layer formed by applying a magnetic coating composition is liable to be roughened due to the roughening of the film surface to deteriorate the electromagnetic characteristics.
For overcoming these contradicting problems of flatness and lubricity at the same time, there is proposed a method using a biaxially oriented laminated film whose front surface and back surface have different surface roughness, one surface being flat and the other surface being roughened for lubricity.
Japanese Laid-open Patent Publication No. 77,431/1990 discloses a biaxially oriented laminated film formed by laminating a film A on at least one surface of a film B, the film A being composed mainly of a thermoplastic resin A, e.g., an aromatic polyester and inert fine particles, the inert fine particles having an average particle diameter of 0.1 to 10 times as large as the film thickness and being contained in an amount of 0.1 to 50% by weight, the film thickness ranging from 0.005 to 3 .mu.m; and the film B being composed mainly of a thermoplastic resin B and inert fine particles, the inert fine particles having an average particle diameter of 0.007 to 2 .mu.m and being contained in an amount of 0.01 to 0.2% by weight.
Japanese Laid-open Patent Publication No. 1,941/1991 discloses a laminated film whose at least one outermost layer is composed of a polyester containing 0.01 to 5% by weight of inorganic particles having an average particle diameter of 5 .mu.m or less and having a Mohs hardness of at least 8. It is described that the above inorganic particles are, for example, particles of aluminum oxide, silicon carbide or the like, and that the above polyester may contain other inorganic particles such as kaolin, talc or heat-resistant polymer fine powder in addition to the above inorganic fine particles having the above hardness.
Japanese Laid-open Patent Publication No. 246,033/0991 discloses a laminated polyester film for a magnetic recording medium, which is a laminate of a polyester layer A having a surface roughness (Ra) of 0.010 .mu.m or less and a polyester layer B containing 0.01 to 4% by weight of crosslinked polymer particles having an average particle diameter of 0.05 to 3 .mu.m and a particle diameter ratio of 1.2 to 5.0.
Japanese Laid-open Patent Publication No. 294,124/1992 discloses a biaxially oriented laminated polyester film having a layer containing 0.05 to 2% by weight of internally deposited particles formed from inorganic particles A having a Mohs hardness of at least 6 and having an average primary particle diameter of at least 0.005 .mu.m and less than 0.3 .mu.m as cores and 0.05 to 2.0% by weight of inert particles B having an average primary particle diameter of at least 0.1 .mu.m and less than 3.0 .mu.m, the ratio (d/t) of the average primary particle diameter (d. .mu.m) of the above inert particles B to the thickness (t. .mu.m) of the above layer being 0.05 to 3.
Japanese Laid-open Patent Publication No. 62,158/1993 discloses a polyester film for a magnetic recording medium, which is a laminate of a polyester layer A containing inorganic particles (a) having a Mohs hardness of at least 8 and an average particle diameter of 0.005 to 0.5 .mu.m and inert particles (b) having a Mohs hardness of less than 8 and an average particle diameter which is greater than that of the inorganic particles (a) and is 0.1 to 3.0 .mu.m, and a polyester layer B containing inert particles (c) having a Mohs hardness of less than 8 and an average particle diameter which is greater than that of the inorganic particles (a) and is 0.1 to 3.0 .mu.m, the inorganic particles (a) being contained in an amount of 0.25 to 5% by weight based on the polyester amount of the polyester layer A and in an amount of 0.15% by weight or less based on the total polyester amount of the polyester layers A and B, each of the inert particles (b) and the inert particles (c) being contained in an amount of 0.05 to 2.0% by weight based on the polyester amount of the corresponding polyester layer A or B, the exposed surfaces of the polyester layers A and B having friction coefficients (.mu.d) and surface roughness (Ra) which satisfy the following expressions (1), (2) and (3), EQU .vertline..mu.d.sup.A -d.sup.B .vertline..ltoreq.0.05 (1) EQU Ra.sup.B .ltoreq.0.030 (2) EQU .vertline.Ra.sup.A -Ra.sup.B .vertline..ltoreq.0.005 (3)
wherein .mu.d.sup.A and .mu.d.sup.B are respectively friction coefficients of exposed surfaces of the layers A and B. and Ra.sup.A and Ra.sup.B are respectively surface roughness of the layers A and B.
In the method using the above laminated film, the thin layer constituting a roughened surface is required to contain a large amount of inert particles to improve the lubricity. As a result, the abrasion resistance in the step of processing the film into a tape deteriorates, and at the time when a magnetic recording tape formed from the film is running, it is liable to undergo scratching and to produce abrasion dust to cause drop-out in many cases.
With speed-up of the tape production process in recent years, the calender and coater conditions tend to be further severe, and it is increasingly important to improve the film in abrasion resistance during the processing step.
On the other hand, for decreasing cost, a metal guide which is not sufficiently surface-finished or a plastic guide is used as a guide post fixed in the cassette, particularly in a cassette for a VTR. However, the surface of this guide post is very rough. Therefore, the following problem occurs in a magnetic tape which is obtained by applying a conventional method for the improvement of film lubricity and abrasion resistance, such as a method of adding inorganic particles of silicon oxide, titanium dioxide, calcium carbonate, talc, clay or calcined kaolin (see Japanese Laid-open Patent Publication No. 57,562/1979) or a method of depositing fine particles containing calcium, lithium or phosphor in a polymerization system but which is not provided with any back coating. That is, abrasion dust and scratching occur thereon in the step of taking it up into a cassette at a high velocity, and as a result, the drop-out level increases.
For overcoming the above problems, there is proposed a method in which fine particles having a high Mobs hardness are added to decrease scratching (see Japanese Laid-open Patent Publications Nos. 306,220/1989 and 185,533/1990).
The above methods show an effect on a guide post that is provided for the purpose of decreasing cost. However, they show little effect on a conventional metal guide which is sufficiently surface-finished. On the other hand, for preventing scratching in using a metal guide which is sufficiently surface-finished, there is also a method in which inert particles having a little greater size than the above fine particles are used in combination. However, there has been found a problem that abrasion dust occurs.
Further, due to the use of particles having high hardness, the edge of a slitting blade is extremely worn out in producing a polyester film or processing the polyester film into a magnetic tape, so that the life of the edge is decreased or the blade sharpness is dulled. This results in an increase of the amount of chips and a deterioration of the cut surface, which are liable to cause drop-out.